The localized heating of tissues may be useful for therapeutic treatments. For example increased tissue temperature may be used to induce necrosis or induce other physiological changes in the target tissue. There exists a variety of means for localized heating of tissue. High Intensity Focused Ultrasound (HIFU), the ablation of tissue with lasers, and the heating of tissue with electromagnetic fields are some examples of how tissue may be heated locally.
When heating tissue locally, it is beneficial to either measure the temperature of the region being heated and/or tissue surrounding the tissue being heated. For instance, during a HIFU procedure it may be beneficial to monitor the temperature of surrounding tissue to ensure that damage to sensitive organs or tissue does not result.
Magnetic resonance imaging has been combined with HIFU to monitor the heating effect caused by HIFU. The combination of magnetic resonance imaging and HIFU is also beneficial because the magnetic resonance imaging may be used to provide anatomical data as well as to measure the temperature within a subject.
U.S. Pat. No. 7,542,793 discloses a MRI system used to guide an ultrasonic transducer at tissues to be treated and to produce images which enable the treatment of tissues to be monitored. Changes in amplitude of a spin-echo NMR signal from a reference spin-echo NRM signal are used to produce image which indicate temperature changes in both fat and water. The method disclosed in this patent is a proton resonance frequency shift (PRF) method of determining the temperature.